Very likely fell into the category of "engineering judgement" .. as in ... "what do you reckon ? .. how about 1.3 as a rough enough place to start ?"
Same thing with the 70mph SE stall limit ... finger in the wind starting point which became enshrined as Gospel.
Similar thing with the 50ft screen height (although that was based, apparently, on tree heights around a particular military parade ground during a demo in the olden days) ...